art collector needs advice about photoshop to get higer resolution

Hi, I collect paintings and am having problems trying to ID the artist's signature.I have a few signed paintings but when I download another painting by the same artist to compare the signature I can't get the picture I downloaded clear enough to see if the signatures match.I have been successful on a few paintings by adjusting the brightness and contrast enough to make a positive id.If I make the image size bigger or try to change the resolution it gets to blurry to read.I have had a little success by inverting the photo I am using to compare but depending on the color of the painting it only works about 25% of the time. I used to do this alot a few years ago when I used photoshop every day and it worked great 95 % of the time but I have been really busy and haven't even used photoshop other than for simple stuff in about 2 years and forgot most of the tricks I used to know.Can anyone give me any suggestions to how I can adjust the photos or sugest any filters so I can get a good look at the artist's signature to compare the signature on my paintings with.Thanks in advance.