To make a good gif, you want to have a standard canvas size for every frame in the gif. All your images should be the same dimensions, or at least close. If you want to put still images in an animated sequence, you should be able to work with something like
https://ezgif.com/maker and it will allow you add the photos, then crop, resize, change the speed of each frame, etc. and put it all into a gif. You may be able to do something similar in windows photo editor, but IDK. I also use GIMP a ton. It's amazing!