Latencies pretty good these days (superior to GPRS), although latency is only an issue when you're doing something interactive (like my line of work....) with file transfers in single direction it's not a massive impact on user perception.
3G is the best performing network for this activity, but the pricing structure is horrendous for large file transfers (but great for my line of work

)
I'd also be dubious about the quality of 3G coverage in the areas you'll be looking to do the transfers from. Once you get out of the major cities and towns it drops right off, so you'd probably end up falling back to GPRS all the time (which is painfully slow).
The prices from Laptop Shop look quite reasonable though it works out to just 75 photo's / month, and coverage would be an issue.
Not sure what the pricing strategy is for the Inmarsat (probably a similar thing) but coverage would be better for your needs.
Jason