>>215
>file formats
If for some reason you like information overload, or are just curious:
https://en.wikipedia.org/wiki/Comparison_of_graphics_file_formats
If you only really care in the scope of this site, heres a short blurb:
http://www.sitepoint.com/gif-jpg-png-whats-difference/
>size
It's up to you really. I'd say a good rule of thumb for any image you're using is resolutions higher than 1600 in either dimension are kind of excessive, unless the image contains a large amount of text. There are exceptions to this of course, if you are using images in the context of wallpapers, printing, if the image itself is the topic of discussion, or if you are uploading an image for further editing by other people. For reaction images, my personal taste is to have them just above the image preview size (which, as far as I can tell, is 255 pixels in either dimension), so if people want a bigger version they can click on it, and view it in page, if they don't it doesn't lose too much effect.
If you mean the size of the file, I don't think it really matters outside of the stuff above and site wide limits (8mb last I checked). If you can make it smaller without losing anything you want to keep, I'd say do it, but if you don't it's likely no one will notice.