How to give credit to a modified image? in a free product

An adobe Stock perpetual, worldwide license allows you to use your licensed asset in all media, including print, presentations, broadcasts, websites, and on social media sites. As long as you will give away fewer than 500,000, then you just need a standard license. (https://stock.adobe.com/license-terms)

A photo credit only has to be added only when using the image in an editorial article. When editorial images are used in print, websites, blogs, etc. you must include the credit line mentioned on the Adobe Stock website and contained in the IPTC credit line field. For example, “Agency Name/Author Name - stock.adobe.com."  

You can edit the non-editorial asset in any way you deem necessary. (Note, however, that your modifications must not violate or infringe on the intellectual property or other rights of any person or entity, nor place the author or the model in a bad light or depict them in any way that might be deemed offensive. See http://www.adobe.com/go/stockterms.)