Interesting problem that I had not looked into for myself. I just sent a text and attached an image for the first time. I got the message that the image was too large... compressing. It sent the image and when I looked at the sent text, the image is shown with my message, but has a Play button on the image! If I press the play button, it shows the image as a 5 second video, with nothing but the image (no motion).
It seems that this is a problem with the SIIX default text app and MMS. It may be related to the compression changing the format of the image into MP4 instead. I will ask my brother (BB user) to let me know what he received on his end and figure out if the image is actually converted into a video format. One thing to note is that my text message was sent instantly, so the slow sending you are experiencing is probably something local, or related to your phone.
Have you reset to default to see if it makes a difference? (NOTE: IF YOU DO THIS YOU WILL LOSE ALL SETTINGS AND DATA!)
I have 2 bugs with my SIIX that are a pain in my side, this is now a 3rd, but I have been waiting to complain since ICS is coming early June. When it is available I will reset to default, erasing everything from my phone, then update and see if my problems are gone.
So... I don't have an explanation, but it seems that the defalt texting app has an MMS bug when sending images.
If my answer helped you, please don't forget to click the "Thanks" button!