Defining images with resolutions or DPI is only useful, when you combine the DPI with a dimension ie width by height at (x) dpi) . A single pixel can be defined as 300 dpi or 72 dpi, it will still remain only one pixel.

Defining an RGB file is a 32 bit file is also problematic as a photographer would consider this to be a High Dynamic Range HDR image and 3D animator might consider it a 8 bits per channel RGB with an Alpha Channel or transparency mask and a print designer might consider the file a 8 bits per channel CMYK file.

It is in fact irrelevant. The browser works in pixels and 100 pixels are 100 pixels no matter what dpi you select. Dpi is only relevant for print, when the driver has to calculate how big a pixel actually are.

72 dpi optimized, 32bit and RGB. There's no need for any higher resolution for an web image.

It depends what the image will be used for, will it be just an on-screen image? If so 72dpi should do the job as suggested by Pablo.

Your image should have a resolution of 72dpi for web. Most applications create images at this resolution by defualt. Of course you can use something like 300dpi, and it will render correctly and print at this higher quality, however, it will take a longer time to load. Which is negative to usability.