Resolution is completely immaterial. 72, 300 - all look the same on the screen. It is only of significance when printing.
The recommendation is for a maximum pixel size of 640 on the longest side - but very few will have a problem with images of say 800 on the longest side and 700 if it is portrait format.
As you have image upload you need to keep the file size to under 200 K - which means that larger images need saving with more compression which can be a problem with some larger images - depending on the amount of detail in them.
