Should I charge?

I need some help with a situation I'm in. Im'm fairly new to the freelancing thing. I've been working on a website for a client of mine, an art gallery. Every month when they have a new exhibition they ask me to add a new artist profile and add artwork images. I charge them by the hour.

I added a new artist profile with her artwork images. The images were supplied by the artist and are photographs of the artworks. I re-size the images and add them to the site. The gallery was happy. Then I get a call from the gallery and they said the artist told them the images on the site are too dark, and asked if I can edit it a bit so that it looks more like the images on the artist's website. So I looked at the artist's website and saw what it is supposed to look like and edited it accordingly. It's almost 40 images so it took some time. Then I get a call from the artist and she said it still doesn't look 100%, and she said because she created the artworks she can see small details that don't look right. So I told her, in stead of me having to edit the images again, to get her website guy to send me the images he used on her site so that I can put it on the gallery site. I received the new images and after confirming with the gallery I added it to their site.

Now what I want help with is what I should charge them for. Should I only charge them only for the time I spent adding the new images? Or should I charge them for all the time I spent working in total, including the editing that they weren't happy with? When they gave me the original images I assumed it was what they wanted on the site, and I haven't seen the artworks before so I had no idea what it was supposed to look like in terms of brightness etc. Only when I saw the images on her site I got an idea of what it should look like, but I couldn't replicate it 100% because the images on the artist's site were also photographs but had bright lights shining on it, and the images that were given to me were just normal photographs without any special lighting.

I don't to overcharge them, but I also feel that they kind of messed me around and the artist could have sent the images from her website from the beginning. What would you guys do? Help and advice would me much appreciated!