Hey Elodie,
Each of those images has the alignnone class which means they don't have an alignment selected. You need to select align left on each of them if you want them to align.
Otherwise you would need to remove the block value for images in the visual editor. Something like:
.entry-content img {
display: initial;
}
I don't know what kind of effects this would have with other elements but its the display:block which is causing images to take up a full line by themselves unless given a float with the image alignment.
Regards,
Devin