I understand so I will provide the answers. It may help others.
In the admin.php file of the Timeline addon I added this:
'image'=>array(
'type'=>'media',
'title'=>JText::_('COM_SPPAGEBUILDER_ADDON_IMAGE_SELECT'),
'desc'=>JText::_('COM_SPPAGEBUILDER_ADDON_IMAGE_SELECT_DESC'),
'show_input' => true,
'std'=> array('src' => 'sppagebuilder.com/addons/image/image1.jpg','height' => '','width' => ''),
),
'alt_text'=>array(
'type'=>'text',
'title'=>JText::_('COM_SPPAGEBUILDER_GLOBAL_ALT_TEXT'),
'desc'=>JText::_('COM_SPPAGEBUILDER_GLOBAL_ALT_TEXT_DESC'),
'std'=>'Picture',
'depends'=>array(
array('image', '!=', ''),
),
),
between the lines 'content'=>array(...) and 'date'=>array(...)
This allows the admin side to have 2 additional fields per item: a file field and an Alt field for the image.
On the side of the site.php file that i overide i have added these lines:
$image_timeline = isset($timeline->image) && $timeline->image ? $timeline->image: '';
$image_timeline_src = isset($image_timeline->src) ? $image_timeline->src: $image_timeline;
$output .= '<img src="'.$image_timeline_src.'" alt="'.$timeline->title.'">';
after the foreach loop to display each image of each item.
I have not yet succeeded in recovering the ALT variable of the image so by default I recover the title of the item.
Have a good day