I have lots of pictures, 700, and lots of text threads. Over 4000 messages saved. If I delete some pictures will that conserve space for the texts? Are they stored in the "same space"? Will freeing up picture storage have an impact on available text storage?
Does the phone support an SD card? If so, move your pics to the card, and autosave them to the card, rather than the phone's internal storage.
As far as using the same "space" - I don't know how the space is allocated. What device do you have?
You might want to look into BitPim and see if you can transfer the texts to your computer for storage, and remove them from the phone, freeing up space for more.