Email support@setdpi.com. That address is for:
Do not attach images. SetDPI never receives, stores, or looks at your pictures — processing happens only in your browser, and this inbox is not a way around that. Describe the problem in words: file type, pixel dimensions, and the DPI you expected. An attachment is not needed, and sending one would break the promise the site makes.
There is no contact form and no account. Mail is the only channel. Privacy requests go to the same address; what the site can and cannot do with them is explained in the privacy policy.