For general images you should aim to upload at least 520 x 390 pixels which is a 4:3 aspect ratio. These include image widgets and news report images.
Website Header Images
Public website header image is 1220 x 190 pixels.
You can also upload a header logo image, this could be your league's crest / logo or it might be a sponsor's image.
This should be 173 x 130 pixels.
You can upload a transparent .png file if you don't want the image to have a background colour.
Division, Cup and Team Logos
Logos for divisions, cups and teams should be 40 x 40 pixels