You can either upgrade to a version that handles the PNG format or hack the upsimage code to use an older version of gd. You really should find a newer browser, as GIF support has been dead for quite some time now. No problems were reported. As for 2038, well, that's another story. My upsstats pages now have broken images where the bars used to be, and running upsimage.cgi by hand gives me something about libgd.so.0 not being found.
Recent versions appear to have dumped configure and have a simple build process again.
Try upgrading gd if you want to be current. If your version is really using shared libraries, run ldconfig or its equivalent to rebuild the cache. Then it should work. It may mean more programs running, but the flexibility you get is usually worth it. The rest of them just run upsmon.