First of all, I'd like to give my thank you to all whom has posted on this forum. The ideas and knowledge that you've shared on this forum has been a tremendous resource to my learnings. Over the past few years, I've worked on numerous images posted by members on this forum to refine my retouching technique. With that being said, there are a few images that I feel that the time and effort that I put in, and the quality of the output is satisfiable to be included in my portfolio. One thing that I've been neglecting to do this entire time is to keep track of the original thread that I got the image from. As a result, I have a bunch of images that I have no clue which thread that they belong. Although I do know a fews where they belong, but I'm not sure if the poster is the original author of the image.
My concern is that is it satisfiable to credit each image with "Image courtesy of RetouchPRO" since each image is going to be use strictly for my personal portfolio?

That would be up to each individual image owner. I can't give permission on their behalf. However, Challenge and Archive images are submitted assuming they'll be used for portfolios. But it's still a good thing to contact the owner.

Thanks for your reply.

I've been trying to search for the original posters for the images I want to use for the past 2 hours, but there's one that I was unable to locate. It'd be greatly appreciated if someone could direct me to the right thread for this image.

right click on the original image and look at the title of the image... the original title. then, simply do a search of the forums using the title name. that shld find the thread.