Ok, thank Alex,

So to change the size of the thumbnails here is what I did.

in /modules/boonex/timeline/template/css/view.css

at the line 244,

replace

div.bx-tl-view-outline div.bx-tl-il-thumbnail img {
width: 100%;
max-width: 100px;
height: 100%;
max-height: 100px;
}

by

div.bx-tl-view-outline div.bx-tl-il-thumbnail img {
width: 100%;
max-width: 318px;
height: 100%;
max-height: 318px;
}

In the database, in the table `sys_transcoder_filters`

Edit field 'bx_timeline_photos_preview'

'Resize', 'a:3:{s:1:"w";s:3:"100";s:1:"h";s:3:"100";s:13:"square_resize";s:1:"1";}', '0'

by

'Resize', 'a:3:{s:1:"w";s:3:"318";s:1:"h";s:3:"318";s:13:"square_resize";s:1:"1";}', '0'

We can make an intermediate choice, but 100 I find that really little