In the future please post images you are talking about on this Forum (or lores versions, with pixelated sections if necessary for confidentiality reasons).
cinziamarotta probably solved the problem already anyway but in this case it might for example make some difference if by
logos that are in white on a black background
you meant logos that are just sharp-edged white areas or whether they include halftones/grays.
Okay... why did I never think of that, I would always just make a new image
Just a wee heads up, but as much as possible, keep everything on separate layers, and save that as a layered PSD file as your master document. That gives you the maximum flexibility. You might also look at using vector tools for your logo, like the Shape layers and the Pen tool. That will give you nicer, sharper graphics that are infinitely scalable. You can also easily change colours.
Good luck and enjoy Photoshop, but prepare to become addicted. Especially if you are using it to make logos and other graphics.