Just adding to the above answer: If you are going to transmit SSTV on a repeater that is also used for voice, you should try to be a courteous as possible:

• Make yourself aware of any popular nets that might use the repeater. Avoid sending SSTV during those times.
• Listen for a least a minute before transmitting.
• Use fast, low-resolution modes that transmit in under 30 seconds, like the old-school 8-second black and white mode or the 12 or 24 second Robot color modes).
• After sending but before releasing PTT, remember to state your callsign. Might be good to also say your callsign initially.
• Always let the repeater drop and wait a good 30-60 seconds between transmitting images. This gives other operators a chance to chime in or protest.
• If anyone protests, stop.

Just adding to the above answer: If you are going to transmit SSTV on a repeater that is also used for voice, you should try to be a courteous as possible:

• Make yourself aware of any popular nets that might use the repeater. Avoid sending SSTV during those times.
• Listen for a least a minute before transmitting.
• Turn down the volume on your SSTV generator, so as to prevent it from being obnoxiously loud to everyone who might be listening for voice contacts.
• Use fast, low-resolution modes that transmit in under 30 seconds, like the old-school 8-second black and white mode or the 12 or 24 second Robot color modes.
• After sending but before releasing PTT, remember to state your callsign. Might be good to also say your callsign initially.
• Always let the repeater drop and wait a good 30-60 seconds between transmitting images. This gives other operators a chance to chime in or protest.
• If anyone protests, stop.

Also, if the repeater is retransmitting on SSB you really should contact the operator first before transmitting SSTV. Remember, SSTV transmitted on SSB is considered full duty-cycle. While the SSB output on a well-engineered repeater will have no problems, others might not.