2
There might be a css statement preventing borders from showing in one of XOOPS's millions (there are about 15 when I looked) of css files.
Have a look through the core, module and the relevent theme files, checking each .css file for a statement that prevents borders.
I'm no expert in css, but I reckon the code that would prevent borders from showing around images would be something like this:
img
{
border: none;
}
Tell me how you get on