I have received an Image from a source I am not sure about, I have edited it to suit my own project, now do I have to cite the main source?
Of course, should you cite the original sources (just use the golden rule: think how you would like to be treated if you were the author of the original source). You may say something like
adapted from < Figure X of original source >
However, as indicated in the comments, you should make sure that you have the right to do so. Note that sometimes it is possible to "read" the data values from an image and create your own image from those (but you still need to quote the source of the data).
I tend to use (author as derived from_________)
Your best bet is to use open sources. For example, when I manipulate maps I use openstreetmaps.com. Their policy explicitly allows for editing.
Otherwise, if the license is unknown, i.e. the author does not specify in any way, then you should contact the author and receive formal permission (this might need to be done with a form), if it is for formal publication.