Instead of slicing your image into multiple pieces in PSP, upload the whole thing in one piece, and then simply use repeat and offset settings in SL to determine which section of it to show on each prim.
For example, if you wanted to do 20 sections, as you mentioned, you're probably going to arrange your slicing into a 4x5 configuration. That's easy. Simply set the repeats per face on each prim to .20 in one direction (for 5 slices) and .25 in the other direction (for 4 slices). Then simply offset the texture on each surface by consectuve incriments of .20 and .25, and you'll piece together the entire image. Not only will this eliminate any visible seams, but it will also make the whole process much easier on the server since there's only one call to download an asset (the texture), not 20. Not to mention, you'll only have to pay one upload fee instead of 20.
A couple of additional tips:
First, it's not a good idea to use numbers like 1200 or 768 for textures. OpenGL requires that textures be measured in powers of 2 in each dimension. Use anythng else, and SL will resize the image for you at the time of upload, and the results are not always pretty. Usually there's a noticeable loss of quality.
To keep SL from auto-resizing your work, always use any of the following numbers for the height and width of your canvas: 32, 64, 128, 256, 512, 1024, or 2048.
Second, before you upload large textures, make absolutely certain they really need to be that big. Poor texture size management is the single biggest source of lag in SL. Wanna know why normal games on your computer get like 60+ FPS while SL gets 10-15? The biggest reason is that those games are optimized to keep the texture load on your video card under a couple hundred megabytes. All the textures in those games are deliberately kept very tiny. SL, on the other hand, being a user-created environment is not optimized at all. People do silly things like put a 2048x2048 texture on a 2-meter sign, when a 128x128 would have worked just fine.
SL is better at taking small images and blowing them up to full screen size than pretty much any program I've ever seen. To see what I mean, rez a cube, and put a medium sized texture on it, say a 256x256. Now zoom in on it so it fills your screen. If your SL window is 1024 pixels wide, you're now viewing that texture at 16 times its normal size, and it still looks just as good. This is one of the areas where SL really shines.
Now, consider that every 2048x2048 sized texture consumes a whopping 16MB or 12MB of texture memory (16 with transparency, 12 without), every 1024x1024 consumes 4MB or 3MB, and every 512x512 consumes 1MB or 768KB. Multiply that by a few hundred textures in viewing distance, and it's pretty easy to see why video cards choke. In poorly managed areas in SL, there can be tens or hundreds of gigabytes worth of texture data on the screen, while video cards can only handle up to a couple hundred megabytes or so.
More reasonably sized textures, on the other hand, barely consume much texture memory at all. A 256x256 only uses 256KB or 192KB, and a 128x128 uses only 64KB or 48KB.
Always consider when determining ideal texture size how much screen real-estate does this image actually need. In other words, what portion of most people's screens will the object occupy when they view it?
If it's something that needs to be sliced into 20 sections like what you're talking about, then chances are the object it's going on will be something like 40-50M wide. In that case, I'd imagine most people won't be viewing the whole thing at once. If it's a 50M wide wall, for example, that people will primarily be standing next to, not viewing from a distance, then they're only going to see part of the texture at any given time. In that case, it's reasonable to make the texure equal to or larger than average screen size, maybe 1024x1024. However, if people are like to be seeing the whole thing at once, there's absolutely no way it's going to require that many pixels onscreen. 256x256 or 512x512 would work just fine.
Always err on the side of smallness with texturing. There's usually almost no benefit in going big. The rule of thumb I usually recommend to people is about 75% of your textures should always be 256x256 or smaller, about 20% should be 512x512, and the remaining 5% should be 1024x1024 or 2048x2048.
Hope that info helps. Good luck.
