SquishPic can convert your SID images to JPEG, PNG, or TIFF.
Conversion to PNG and TIFF yields a pixel-perfect copy; the output file is not compressed and no image data is lost from the original.
JPEG output yields an image with a 75% quality level. This will likely result in some pixel loss but should be acceptable for most preview purposes.
In all cases, the pixel dimensions of the output file are the same as the input file.
To convert a file:
Open the source image in SquishPic.
Click Decompress Image.
In the Decompress Image dialog, click the desired format.
Each option displays the approximate size of the resulting decompressed file.
Click Decompress, then choose a destination folder and file name and click Save.
You can set the default decompression format and file location in Preferences.