After looking at the shading language from the link you gave me and watching the Intro to shaders in Godot 2 video i finally understood where i was wrong. I couldn't show you any code because i didn't had one ;)
Now having an image as Base and a displacement map image as Map i have this as a result :
The code i've written is for the Fragment part :
uniform texture base;
uniform texture map;
uniform float maximum=0.05;
vec4 displace = tex(map, UV);
float displace_k = displace.g * maximum;
vec2 uv_displaced = vec2(UV.x + displace_k,UV.y + displace_k);
COLOR = tex(base, uv_displaced);
Now i have done it by having an extra input (the Base image) while i would like using a Sprite's Texture directly.This could be done by using a backbuffer as stated in the Screen-reading shaders docs so ... more things to study :)
Thank you for your help memer !
I've included my project file if anyone wants to see it.