I got several special effects in the Marketplace that are PNG files, but I can't find a way to use them in DAZ, and I sure don't have the money to fork out for Adobe Photoshop just so I can add them to an already rendered image.  Any suggestions would be helpful.

A PNG file is a standard image format that can used by any good 2D program including the free GIMP.  Png files can be applied directly to objects from the Surface tab in Studio.

create a new primitive plane

and apply the png file

you may need to also create a transmap for it in any free paint program,

unless it also came with a black and white image to match the color version then this can be used in the opacity

i do this with fire, smoke, fog and fairy sparkles all the time.

While DS can use severalof the "popular" image formats, the shaders that come with it prefer 24 bit image formats like JPEG as they can only see the RGB values in the image and ignore the Alpha.

For that you either need to us it as an overlay with Layered Image Editor (built into DS4), or use the ShaderMixer to access both the RGB and Alpha values.