How to center the profile picture on my Sharepoint list gallery view card?

Peki72 Try removing the "display": "block" styling added for your img element in the JSON.

Here's the working JSON in my case without display styling for the img element: 

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/tile-formatting.schema.json",
  "height": 150,
  "width": 254,
  "hideSelection": false,
  "fillHorizontally": true,
  "formatter": {
    "elmType": "div",
    "attributes": {
      "class": "sp-card-container"
    },
    "style": {
      "text-align": "center"
    },
    "children": [
      {
        "elmType": "div",
        "attributes": {
          "class": "sp-card-defaultClickButton"
        },
        "customRowAction": {
          "action": "defaultClick"
        }
      },
      {
        "elmType": "div",
        "attributes": {
          "class": "ms-bgColor-white sp-css-borderColor-neutralLight sp-card-borderHighlight sp-card-subContainer"
        },
        "children": [
          {
            "elmType": "div",
            "attributes": {
              "class": "sp-card-displayColumnContainer"
            },
            "style": {
              "text-align": "center"
            },
            "children": [
              {
                "elmType": "p",
                "attributes": {
                  "title": "[$Title]",
                  "class": "ms-fontColor-neutralPrimary sp-card-content sp-card-highlightedContent",
                  "role": "heading",
                  "aria-level": "3"
                },
                "style": {
                  "text-align": "center"
                },
                "txtContent": "=if ([$Title] == '', '–', [$Title])"
              }
            ]
          },
          {
            "elmType": "div",
            "attributes": {
              "class": "sp-card-previewColumnContainer"
            },
            "style": {
              "text-align": "center"
            },
            "children": [
              {
                "elmType": "div",
                "children": [
                  {
                    "elmType": "img",
                    "style": {
                      "border-radius": "50%",
                      "margin-bottom": "25px",
                      "text-align": "center"
                    },
                    "attributes": {
                      "src": "=getUserImage([$PersonColumn.email], 'M')"
                    }
                  }
                ]
              }
            ]
          }
        ]
      }
    ]
  }
}

Output: 

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