Best free image optimizers to reduce file size
PNGGauntlet will allow you to optimize (reduce the size of) PNG files. Most important is that no information is lost during this process. PNGauntlet can also be used to convert JPG, GIF, TGA, PCX, and BMP files to PNG.
RIOT (Radical Image Optimization Tool) will help you optimize your jpg, gif and png images even in batch processing, previewing the resulting images. RIOT offers also a plugin for IrfanView. It does not need file extension to recognize format, including support for uncommon types (up to 128 BPP, integer and floating point. EX: hdr images, 16 bit grayscale, etc). Adaptive logarithmic tone mapping algorithm (Drago) is used for HDR images. You can see instant results including resulting file-size and decide if you want to keep metadata (comments, IPTC, Adobe XMP, EXIF profiles, ICC profiles). Common tools: pan and zoom, rotate, flip, resize image by using well known resample filters (ex: Lanczos3, Catmull Rom, Bicubic, and others), in-place compare function (alternativelly display the original image over the optimized image to notice small pixel changes).
Trout GIF Optimizer allows you to reduce the size of a gif file without noticeable changes in its appearance. You can preview the result of the optimization before saving. You can also change the background color of the Preview panes, in order to simulate your publication’s environment. You can apply color reduction selecting between various methods, including palette.
OptiPNG will help you optimize PNG files without losing significant (perceptible) information. OptiPNG will also perform some auxiliary functions like integrity checks, metadata recovery and pixmap-to-PNG conversion. Valid PNG files that cannot be optimized by this program are normally left intact, their size will not grow. The user may request to override this default behavior. Input files are raster image files encoded in PNG format, or even in GIF, BMP, PNM and TIFF formats.