See if you can find a program caled Drop Image. I am using Drop Image 2.5 and it does the job well. Drag, drop and save as. It depends on what type of image you are trying to read on the pc. If you save a file on the mac, you must also include the appropriate file extension otherwise the pc will not recognise the file at all.
Most file formats are cross platform like jpef, tiff, bmp etc.
The mother of all applications for this is Adobe Photoshop which is quite costly and complicated for most users. There are many freeware programs that will work nicely with one or more of these formats. I work in publishing and have to deal with these cross platform issues on a daily basis. Net is the opinions of its users. Such opinions may not be accurate and they are to be used at your own risk.