[Png Photo upload is very dark]

It's safer in Photoshop to use Image --> Duplicate and check the Merged Layers Only box. That way you get a flattened copy and preserve your original in layers. Too many times, I accidentally saved my flattened PSD   (woops!!)

[Png Photo upload is very dark]

Emphasizing what Lindal said, 'Make sure that the texture COLOR is the default white.  Anything else will darken (and/or tint) your photo texture.'

You need to place textures on a white prim face to see them in their full color. If a prim is dark, Full Bright will have little to no effect. You use Full Bright so the picture shows up regardles of time of day (i.e. at midnight, it will still be bright as ever).

You can use PNG or TGA, but you don't need any transparancy. You can even use JPEG (jpg), but many say you are going to see a loss in quality, however I've had no problems when I set the quality slider to maximum. Best to experiment and see what is suitable for your purpose.

[Lines and prisms of all shapes jumping on the screen.]

Not sure it's dual-core as I have I7 and dual 4870x2 & Vista 64-bit with the same problem. Even with latest video drivers I have to disable VBO. My gut feeling is ATI doesn't care about OpenGL as much as DirectX and puts their efforts there, to support gaming.

[Will Second life Work On My Mac?]

You can open prefs before logging in; Ctrl-P. Get things to their very basic before logging in. Select the Graphics tab and click Hardware options. Make sure Anisotropic Filtering is off, disable Antialiasing and uncheck Enable OpenGL Vertex Buffer Objects (likely the culprit). You might want to run texture memory to maybe just 50% of available  just for now. When the settings take effect, your screen may glitch for a bit or show weirdness. Just note where the OK button is and click it and It should clear up. Go ahead and log in now and see if that helps. Once comfortable with everything, you can go back and fiddle with these settings to get better performance or find what freezes it.