I have GIMP and I know the tools for cropping, moving, drawing, making a border, text, and a few other things. The best thing to do is look up exactly what you want to do on google because some things (like just making a border) involve several steps. You can also look up free fonts to download and add to your collection of fonts or patterns.
I actually just learned how to do this today, but it's how you remove part of a picture. I attatched a before and after picture so you get the idea. But heres what you do:
1. Upload picture
2. Select the "clone tool" (it's toward the bottom of your toolbox)
3. Hold down control and select an area you want to duplicate. (So like if you're taking out a horse and you want to replace it with grass, you'd select a grass area.)
4. Click on the areas you want to replace with what you selected. You just have to play around with it to get it right.
5. Once you've finished, use the smudge tool (also at the bottom) and blend the edges together as needed so it looks real.
You can also make the subject of your picture in color, and make the background black/white. You can also make the background sepia/with other colors. Just select what you want instead of "desaturate"
1. Upload photo
2. Go to Layers>Duplicate layer. (Also do control L to pull up the Layers menu)
3. Go to Colors>Desaturate
4. Then, Layer>Transparency>Add Alpha Channel (this makes the top layer clear)
5. Click on the Eraser tool from your toolbox. You can change the brush size by using the Scale option.
7. "Erase" the top layer where you want the color to shine through.
8. If you are done with the picture, you can then go Image>Flatten Image, to merge the two layers. You can then save it. You can also save it when it's double layers, you'll just have to select "export" somewhere along the saving process. Also, make sure you do "save as" not just save because save replaces the original copy of the photo on your computer.
I attatched the same picture above with a really quick version of changing the background. (hense the green around the mane and tail ;) )