If Leonardo DaVinci were to paint Mona Lisa today,he would need a release before licensing her image.
If all you've done is display the girls' photo at a gallery, a release probably isn't required.
When you need a release. A properly drafted release protects you from two types of lawsuits: (1) if you use the girls' image to sell or endorse something; or (2) if you use the image in a way that defames, or invades the subjects' privacy. By signing the release, the subject promises not to sue over these uses. A release is usually not required for informational or artistic uses (that is, those uses considered an expression of free speech).
Going forward. If you plan on making other uses of the image, it's not too late, to get a model release. If the women have reached the age of majority, they can sign the release. If they're still minors, a parent/guardian should affirm the contract. You can download free model releases at our crafts law site.