You need some RegEx for that.
Using the new conditionals operators of EECMS version 2.9:
{if event_photo}
{exp:imgsizer:size src="{event_photo}" alt="{title}" width="300" height="450" class="displayBlock" quality="100"}
{if:elseif url_title *= "keyword"}
{exp:imgsizer:size src="{other_photo}" alt="{title}" width="300" height="450" class="displayBlock" quality="100"}
{if:else}
<img src="/images/_pages/_global/placeholder.jpg"/>
{/if}
Using switchee, if you can't update EECMS:
{exp:switchee variable="{event_photo}/{url_title}" parse="inward"}
{!-- if the concatenation of both variables ends on "/", but has something before it --}
{case value="#./$#"}
{exp:imgsizer:size src="{event_photo}" alt="{title}" width="300" height="450" class="displayBlock" quality="100"}
{/case}
{!-- if there is "keyword" --}
{case value="#keyword#"}
{exp:imgsizer:size src="{other_photo}" alt="{title}" width="300" height="450" class="displayBlock" quality="100"}
{/case}
{case default="yes"}
<img src="/images/_pages/_global/placeholder.jpg"/>
{/case}
{/exp:switchee}