Maths formulas as ugly gif images included in pdf

Hello
I want to convert into pdf xml documents that often include maths formulas, handled as gif images. 
The output looks ugly in 75% of the cases : the images are displayed in the pdf with a big black underline in most cases, in a few cases they turn up OK (I could send a pdf extract as attachment).

For instance, with the fo reproduced below, the image023.gif appears with a black underline, wherease the image025.gif is OK.

<fo:inline>
	<fo:external-graphic src="../objects/image023.gif"/>
</fo:inline>
We have V= 
<fo:inline>
	<fo:external-graphic src="../objects/image025.gif"/>
</fo:inline>

I checked the images, they are correct (the underline is not in the image). The mistake always happens on the same images.

FYI, my documents include as well many *.png images (as fo:block, as reproduced below) without any problem.  
<fo:block text-align="center"><fo:external-graphic src="../objects/ndpOverTime.png"/></fo:block>

I can't include the images that are indeed math formulas as fo:block, because they are sometimes in a sentence (I don't want line break after a math image).

Would anyone ever had encountered something similar before ? I don't understand where the pb comes from, and I don't understand why it does not always (or never!) happen.