You can get an Fl_RGB_Image object from a .png file with:

    https://www.fltk.org/doc-1.3/classFl__PNG__Image.html#a64b3f34f7b97784763802e39e2db3fc2

and similarly with .jpeg:

   https://www.fltk.org/doc-1.3/classFl__JPEG__Image.html#a8a12107808d6e86cd82b7bede8637aeb

and .bmp:

   https://www.fltk.org/doc-1.3/classFl__BMP__Image.html#a1abb2ebb6ca113fd03a967b66624c868

In all these cases, the resulting Fl_RGB_Image object, supposing you call it img, will tell you

if its RGB (img->d() == 3) or RGBA (img->d() == 4)and its width (img->w()) and height (img->h()).
In some cases, the row length is bigger than img->w()*img->d(). In that case img->ld() is != 0

and gives that length. Finally, you get the address of the first byte of the RGB or RGBA data

by img->array of type const unsigned char *. That should put you in the position to build a texture.