I intend to process RAW images of planets clicked by HST,Voyager 1 and 2, Cassini and Gallileo spacecrafts using a few softwares. I am in search of where I can get these raw images from.
I think your best bet for a uniform access experience to the various datasets is the Outer Planet Unified Search (OPUS) system at the Ring-Moon Systems Node of NASA's Planetary Data System which is the official repository for NASA planetary mission data.
The OPUS link of the Ring-Moon System Nodes' front page will bring you to a form that looks like:
For Cassini and Voyager data, you will want to tick the 'ISS' options under 'Instrument Name' as these are the Imaging Science Subsystems for the optical (I'm assuming this is what you want, rather than ultraviolet or infrared imaging). For Galileo, this will be 'SSI' (Solid State Imaging) instead. For Hubble (HST) there are a variety of instruments available that have been used over the years; the Instrumentation page at STSci has links to more information for the current and previous HST instruments. More detailed information about the instrument and the data processing done are at this link for the Cassini ISS, and here for the Voyager ISS; I'm sure there is an information page for the Galileo SSI but it wasn't turning up easily.
For 'Observation Type', you will want to select 'Image' as the type and then you can select a specific body as the target under the third section under the 'Intended Target Name'; the plus symbols for the each planet can be unfolded to narrow down to a specific moon or the planet itself. There are additional constraints available on the left hand side if you want to narrow down by exposure time or wavelength etc. Once you have narrowed things down to how you want them, you should be able to click on the 'Browse Results' link in the top bar (which should have a much smaller number in blue than in the screenshot above) to see thumbnails and basic details of the results and "add them to cart" to download them (if you want to download the entire result set, you will want to click on the 'View Table' option in the top left which gives a "add all to cart" option.
There is a 'Getting Started' guide under the Help menu on the OPUS page and also a link to the API guide as well if you are more comfortable with programmatic access to query and download the data, which could be easier if there are large volumes of data to download. Finally, depending on your level of experience with these sorts of data, you might be interested in Emily Lakdawalla's free course on the Basics of Digital Imaging.