What are the possible ways to detect skin while classifying diseases?
You have to have sample tissues from skins with Melanoma, Psoriasis and Healthy Skins.

As is understood in the question, you developed a solution which can distinguish with 96% accuracy between melanoma and psoriasis. You are missing from the possibility of distinguish between sick and healthy skins.

The model you are looking for is a multiclass classification. Where CLASS3: Healthy skin.

You are doing ok so far, you just need to include this category and images of healthy skins in your model.