Keep in mind that some outdated UENI plans have limited Gallery space. If you haven't yet, upgrade your plan here.
UENI recommends the following resolutions:
Logo: minimum size 200x200px
Header image (big banner image at the top of your website): minimum size 1900x500px
Gallery images: minimum size 1024x300px
If you're using a Windows operating system, use this third-party tool to resize your images.