The iMac does not officially support any other resolutions. However, you can cheat a bit.
Download SuperRes 1.1
from Griffin Technology. It lets you choose extra resolutions, and it's free (unlike other utilities of this type). The SuperRes control strip module is not compatible with OS 9.1 - it will prevent the control strip from launching, but it works fine under earlier OS versions. And the Control Panel still works under 9.1. Select from the All
resolutions list in the control panel.
You can also cheat a little more, if that does not do the trick. You can download Wish I Were
. This is a utility that tricks the OS into thinking it's running on a different model of Mac, you can choose which model. Try a beige G3 or a 9600, etc. See if SuperRes can select 832x624, that's the only in-between resolution you might be able to reach.
If none of these work, Search the iMac Forum. Someone did manage to reach an unsupported resolution with some tricks, I believe it was 1152x870. They wanted the higher resolution for a large external monitor. Reaching 832x624 should be a lot easier.