Gimp is your best one to learn, in my opinion, it's way more effective than paint, and things like Digital Pro tend to use techniques that when you learn them, the skills don't help you with other graphic programs. Paint is for drawing your own, and Dig Pro is just for pictures, Gimp will let you do either one, it's a good versatile program. .
Looking at my computers, looks like I got rid of Gimp for space reasons when I dumped the Adobe Creative Suite 5.5 in.... 18 big programs, that do everything. I had to clear a LOT of room! Looking at screenshots, looks like they are still doing basically photoshop, so I'll tell you what I'd do in Photoshop CS2, which is a good easy one that looks like Gimp, and see if you can find the same things in Gimp, I think you can.
When I say things formatted as file/new/ 800 x 600 pixels it means go the tab named File, then down the New option, then input the numbers into the box it will give you. Just the easy way to write it all. The Control key is abbreviated cntrl, I keep typoing it, been using cntrl so long I forget to add the vowels :)
To put two pictures
side by side :
Open both pictures. Let them sit there for a minute. (I'm going to assume they are 5x7, just for an example)
File/new/5 inches x 14 inches. This will make a blank page that is big enough for both. If you want a border and a space between them, make it 6x 15.5, that gives .5 inches on each side, and .5 inches between them.
Click on one of the pictures, Hit Cntrl A (which means "select all') then Cntrl C (which means "copy everything that is selected")
Move to the blank page, Cntrl V (which means Paste whatever is on the clipboard) The picture is now sitting there, move it where you want it.
Go to the other picture, cntrl A, Cntrl C, move to the blank, cntrl V, move it where you want it.
Each picture is on a separate layer, which is a magic word in graphics, each layer can be moved, or changed, separately. The
should be a box on the screen (usually lower right corner area) labeled Layers, which has three layers showing right now, "background, layer 1(the first picture) layer 2 (the second pic)" if you don't have a box that looks like that, go to Window tab and check the Layers option, or Show Layers, should make it appear. It might be under View, depending on your version.
When you have the layer box showing, click on a layer, that selects it, that means whatever you do will happen to that layer, and only that layer. You can also right click on it (just hover the
mouse over the layer you want, and right click) a menu with all kinds of things comes up. This where you can rename it (I currently have a thing I'm working on that has over 80 layers, I name them, or I'd NEVER find them!) delete the whole layer, duplicate it, merge it with another layer, etc. Right now, you want to put it exactly where you want it. Select the layer, and use your mouse or the arrow keys to move it exactly where you want it. Hit your Enter key on your keyboard. If it won't do the next thing you want, always click enter, some things require Enter to be finished, and it saves getting annoyed at it if you try that first.
Move to the next layer, select it, move it where you want. Does it look right? If not, go back to the first one, tweak them back and forth till you get what you want. Cntrl Z will undo the last thing you did, it will save your tail!! Alt-cntrl Z (all at the same time, takes three fingers) will change back things going backwards, the last things you did. Keep hitting that combination, and it will walk you backwards through everything you did.
At this point you can save it in layers, or flatten all the layers down, and save it as a JPG (or other formats). To save it as is (so you can mess with it later) cntrl S (save) it will give you a box where you can save itwhere you want it, name it, save it in various formats, if you don't know what you want, accept what it offers. Hit Ok. To save it as done "Layer/Flatten Image" now all your layers in the box are all in one layer. Cntrl S and choose Jpg, name it. Hit Ok
That file now has two pictures exactly where you put them!
It's more complex to type than to do, I could have done it in less than 1.5 mins, it's taken me 20 to type it. It gets easy fast. I REALLY vote you learn Gimp, as it's like a lot of the bigger programs, and when you are ready for a better one (if you want) you'll have transferable skills :)
There are tutorials on line, for both Gimp and Photoshop. if you use a Photoshop tutorial for Gimp, a Photoshop CS2 tutorial will probably be most useful. If you get into the higher versions, they get very complex, and assume you know a lot already.
If you have problems, tell me exactly where you are stuck, and I can get you where you want to be :)
Graphics is a fun rabbithole, only scary for the first few days. Then it's a blast :)
Suggestion, pick a picture you don't care about and torture it, you learn a lot that way!