use Alpha from image in Fragment shader

Hey, i got an image with RGBA data (was a png), but it seems like im not able to archieve that i just can blend this with its alpha channel.

i tried so much from the forums here:

 Tags { "RenderType"="Transparent" "Queue"="Transparent" }
 ZWrite On    
 Lighting Off    
 ColorMask RGBA    
 Blend SrcAlpha OneMinusSrcAlpha and even Blend SrcAlpha SrcAlpha

but they all seems to change nothing at all.

i would like to let it look like i would use glblendfunc(gl_src_alpha gl_one_minus_src_alpha) in good old openGL i dont want to waste another memory for an Alpha map-image wich is just grey or something like this.

so is there any advice how to archieve this in general within the frag shader?