Are you pushing the home key after receiving a picture? Try hitting back, instead. I notice the same lagginess when I keep getting SMS with Google Voice. This is what I did to create the lagginess:
Get a text
Pull down the notification bar
Tap on the message, read, respond.
Hit home
Got another text
repeat (notification bar > message > respond)
hit home
Got another text
repeat
I did this a few times and after 5 or 6 messages, the phone lags when opening GVoice. So I looked in the Running Programs tab of Manage Applications and I found that GVoice was opened 6 or 7 times in it, eating RAM like crazy.
So, my solution was to hit the back button instead of home when I was done with GVoice, or any app for that matter. And sure enough, I saw GVoice come up and close every time I hit the back button until only 1 instance of it existed. Now I use Home as a multi-task button and Back as a 'quit' button. Give it a shot, see if it helps you.