I presume by imageready you really mean imagemagick.
Your code is bit confusing. It is usually better to put in explicit values and image names and post a link to your image, so that others can test.
convert $1 -gravity southeast -pointsize $largeur/100 -fill white -annotate 90x90+$largeur+$hauteur 'TITRE' test$1 'TITRE' test.jpg
What is $1
What is test$1
And why do you have
test$1 'TITRE' test.jpg
I presume test.jpg is the output. But why have you repeated 'TITRE' twice and why the test$1
Getting back to your original question, you might be better using -gravity and the annotate arguments are then supposed to be justified relative to the gravity. That seems to work fine if you don't rotate the text. For example
convert logo: -gravity south -pointsize 48 -font Arial -annotate +0+20 "THIS IS A TEST" logo_annotated.png
This centers the text relative to the bottom center and shifts it up 20 pixels vertically.
But it seems to break down when you rotate the text, so that it is not centered in one dimension.
convert logo: -gravity west -pointsize 48 -font Arial -annotate 90x90+45+0 "THIS IS A TEST" logo_annotated.png
This should do the same relative to the left side of the image. It shifts it right by 45 pixels, but does not center the text vertically.
I am not sure if this would be considered a bug or not, because it seems to get confused by the rotation.
If you use -gravity center, then all the justification should be relative to the center of your input image. So this works fine.
convert logo: -gravity center -pointsize 48 -font Arial -annotate +0+0 "THIS IS A TEST" logo_annotated.png
convert logo: -gravity center -pointsize 48 -font Arial -annotate 90x90+0+0 "THIS IS A TEST" logo_annotated.png
But it also fails when rotated for -gravity north and -gravity east.