Using css class selectors on a specifc page [closed]

This is off-topic, as it’s a CSS question, not a WordPress question. That being said.. You need to attach your class to your tag. Use:

img.activities {
    /* stuff here */
}

Your current CSS selector targets this:

<span class="activities">
    <img src="https://wordpress.stackexchange.com/questions/145611/image.png"/>
</span>