Seems difficult to help you without actual code, but:
Just adding the attribute "align=center" does not help (see http://www-db.deis.unibo.it/courses/TW/DOCS/w3schools/tags/att_object_align.asp.html ) - you need to set the correct CSS, which could be "text-align: center" on the wrapping div, as you can see here: https://css-tricks.com/almanac/properties/t/text-align/
Otherwise, you have plenty of options when centering with flexbox, as you can see here: https://developer.mozilla.org/de/docs/Web/CSS/CSS_Flexible_Box_Layout/Aligning_Items_in_a_Flex_Container
You can use Inline-styling to provide that code (use <style>
-tag>)
If you use any special framework, which parses HTML attributes, then this also needs to be specified in the question.
Hopefully this answer does provide at least some value.
Next time, try to provide a code-snippet - this makes helping you much easier.