I've got the EGS 24/28 in my Amiga 2500 triple booting EGS, Cybergrafix and Picasso 96 drivers. The Picasso 96 utility in the Prefs drawer should list your card and have a selection of video modes for it. There is an option to test these modes and I would start by testing them all. In the test mode, if it works, you will get a test pattern on your screen and if it doesn't, you just have to click the mouse button to return to a native amiga mode--or a Picasso 96 mode that works. I am using a 15" Phillips LCD monitor with it. When I installed the Picasso drivers, I got 640x480 and 800x600 modes that work but I still haven't been able to tweak the 1024x768 mode to work. Usually you just have to adjust the clock rate but sometimes you really have to get into adjusting vertical and horizontal blanking intervals to get things to work. I had to do this with my Merlin card on my A4000. Also, which version of Picasso 96 are you using. There are several versions floating around the net. The latest one--I think--is 2.0. The good thing about the EGS spectrum card is that you can use Cybergrafix as well. Cybergrafix works better for some things--like Deluxe Paint works in RTG modes with Cybergrafix. And last but not least are the underrated EGS drivers which can only do a 256 color workbench but the EGS workbench is fast and stable and feels much more like a native Amiga workbench and Ad Pro can render a 24 bit full screen image in a matter of seconds in EGS--I used to pass 24 bit images from my A2500 to my roommates 486 PC and the PC which was in theory faster than a 68030 Amiga, took about a minute to render and display a 24 bit image.
The cable you need to pass Amiga Video to the EGS card is a 23 pin to 9 pin female cable. I think there are some Amiga monitor cables in this configuration. I have a 2320 flicker fixer and had to make a 15 pin vga to 9 pin female cable. 15 pin male to 9 pin male cables are common but 15 to 9 pin female ones just weren't used that much