I would probably back off to 1/4 of the rated power as an absolute max
since the amp is rated 600 watts on SSB. I probably would run it at 75
or 80 watts for SSTV to let it run at less than flat out.

For the short time the pix is being sent the amp should take atleast 500
watts out if only only one pix is sent as per usual. It only takes 10
seconds or less to send the pix.
If using RTTY or PSK31 where the amp is keyed up for a lot longer time , it
should be derated alot.