MySQL 8.0

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create table t as 
    select 'x' as name, 'name=hello; image=xxx' as image_description union all
    select 'y' as name, 'name=hi; image=yyy' as image_description union all
    select 'z' as name, 'name=by; image=zzz' as image_description
    
select substring_index(image_description, 'image=', -1)
from t;

substring_index(image_description, 'image=', -1)

xxx

yyy

zzz

select substring_index(substring_index(image_description, 'image=', -1), ';', 1)
from t;

substring_index(substring_index(image_description, 'image=', -1), ';', 1)

xxx

yyy

zzz