Image processing requires a lot of CPU power. That means you need high powered computers with their electrical consumption and heat generation problems. An even bigger problem is radiation hardening. There's a good reason why the CPUs on the Shuttle are little more powerful than old 80486s. Densely packed integrated circuits are prone to single event upsets (SEUs) from radiation. These SEUs are enough to flip bits, causing random errors. Satellites and spacecraft use radiation hardened chips to reduce the problem, but these chips are very expensive compared to commerical products. You also need to spend the extra time on fault-tolerant programming to trap the effects of bit flips so they don't generate errors or cause the system to crash.

There's also the problem of antennas on the ISS. If you're talking about image processing from a series of satellites (or the absurd notion of lunar observatories to detect NEOs), then you're talking about enormous bandwidths. The ISS is a moving target so your receive antennas (or laser sensors) would need to be able to trackthe transmitter. Unless you put some relay satellites out around geosynch, you'd be out of view of the transmitter for much of each orbit. For RF antennas, bandwidth is still heavily driven by the EIRP of the transmitter antenna and the gain of the receiver system. Even something as big as the ISS has limits when it comes to how big an antenna you can hang on it. Moving the antenna to track the source would cause vibrations and other issues. If you go with a phased array antenna, you aren't going to get the gain of a good dish so your data rate is reduced.

Ground based antennas aren't nearly so limited by physical size. Increasing the antenna's gain by increasing the diameter or decreasing the beamwidth is relatively simple. You don't have to worry about SEUs and power/heat issues effectively disappear. Tracking satellites from the ground does have limitations when it comes to visibility but there's no technical reason why the search satellites couldn't be out at geosynch so they're always in view of the ground stations.