This would be very handy if it were possible from Collector. From what I have read, and anyone please correct me if I'm wrong, you have to change the resolution on the device. But this isn't possible to do in the settings of an iPhone (brilliant) so you have to buy an app that allows you to adjust the resolution of pictures and take the picture first then choose from library.
You should not need another app or settings, if at all possible Collector should use the same mechanism as the Mail app to prompt the user to choose a size of the image to attach. This functionality has been in the Mail app for as long as I can remember. Jeff Shaner