I'm a little suspicious about this snippet:
FREE_IMAGE_FORMAT formato =
FIBITMAP* imagen = FreeImage_Load(formato, imageName);
FIBITMAP* temp = imagen;
imagen = FreeImage_ConvertTo32Bits(imagen);
If your .png is authored and is loaded correctly, then it should already be a 32 bit image. I wonder if it's only 24-bit and FreeImage_ConvertTo32Bits is adding an empty alpha channel for you.
Also, could you post your fragment shader, please? The line "I should note as well that if I change the Fragment shader color value to vec4 (it's currently set to a vec3) and include an alpha color (set to anything between 0.1 and 1)" sounds a bit wrong, you probably should be dealing with vec4s when you're manipulating rgba colours.
I suppose that to narrow down whether it's a image loading or a rendering problem, you could experiment by modifying the "textura[j*4+3]= pixels[j*4+3];" line to "textura[j*4+3]= 128;" and see if that gives you a semi-transparent image. If so, then you know it's your image loading, otherwise it's your rendeiring. (Actually maybe it's textura[j*4+0] which represents alpha, I can never remember which order these things are supposed to be)