You should look at the sticky post "All Things Root Guide" in this forum. It contains a number of Desire-specific guides and FAQs, including a more reliable guide to
rooting using revolutionary.
Anyway, the good news is that this is fixable, though I expect you have lost some data.
First, can I check what exactly you did? You wiped the phone, and you didn't take a backup from recovery (the person who wrote that this is optional is an idiot, pure and simple!). Did you take any other backups before attempting to flash? Not that that changes much here - they key point is whether you wiped the phone, because if you did then I don't have to worry about coming up with a clever trick to save your data because they are gone, and you either have a backup or you do not.
So I'm not sure what you did wrong. If you tried to flash the ROM that guy linked that would be a bad mistake, since ROMs are device-specific so a Wildfire ROM absolutely won't work on the Desire. But the message you've photographed means that the ROM is incompatible with the custom recovery that revolutionary installed, so it ought not to have flashed anything. But unless something was flashed I don't know why the phone won't boot - this is where knowing exactly what you did would help.
You could try a factory reset from recovery again and see whether the phone will boot. But my guess is that your ROM is toast, in which case you need to flash a new ROM.
There is a large collection of Desire ROMs
here. If you want one close to what you originally had I'd recommend either teppic74's rooted stock
Froyo ROM or his
Gingerbread one. You install either by placing the ROM .zip on the SD card (you can do this using a card reader, or mount the card on your PC using the "mounts and storage" menu in recovery), then factory reset the phone and select the option to install zip from SD. Do read any instructions in the thread, but don't worry about flashing a new radio even if the thread says so (I'll add a note about flashing radios at the end). If you would prefer to just go ahead and install CyanogenMod then you'll find a link to a Desire version there too. Do read the first post of the thread, so you know how to add Google apps (such as the Market/Play Store), because these aren't included in CM. Avoid anything using Sense 2 or higher, because those need your SD card to be set up in a particular way and you don't want to deal with that right now. Personally I'd get the rooted stock ROM on, get the phone working, then take a deep breath and do a bit more reading before moving on to fancier stuff.
Now an older ROM might give you the same error you photographed. If so, you want to change recovery. Our own revolutionary guide, which incidentally recommends you don't use revolutionary's recovery, will tell you how to do this (points 2e, 3 and 6 of the guide). ClockWorkMod 2.5.0.7 will work on your phone (I'd personally recommend 4Ext Touch, but if you want to flash an old ROM then CWM 2.5.0.7 is good, and will work with newer ones too).
Right, before my browser crashes again and I have to type this once more, a final warning about flashing radios. You install a radio .zip in the same way as a ROM: "install zip from sd card", select zip, and go. But a bad radio install will kill your phone for real. So always check the md5 code of the radio file before installing - the All Things Root Guide sticky post has a link to a post about this. This is to ensure you have a good download. Then don't try to install a radio with a low battery, as you must not interrupt the process. Flash it, which is quick, then select the reboot option in the recovery menu (don't try to install anything else at the same time). The actual install happens when you reboot, and will result in the phone rebooting itself a couple of times, so don't panic! I would only flash a new radio once you have the phone working, and then only if you have a problem with reception or wifi - it's not something to try when the phone or owner is in a distressed state
DIDN'T TAKE A BACKUP
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